Will there ever be Achievement for older games?

No

Probably not:

https://www.google.com/search?q=gog+achievements+for+older+games

One of the many threads on the topic:

https://www.gog.com/forum/general/achievements_on_old_games

Achievement is popular enough for console retro gamers.
For example: http://retroachievements.org

In fact, GOG could do it quite easily. Just use a patched DOSBox or ScummVM, and hundreds of games will have achievement immediately.

But no, I doubt GOG will do it soon.

And how is that supposed to work? Achievements have to be implemented in the game, and then (if supported) the client running it is notified. The old games simply had no achievements, to implement them you would need the source code and basically make a port with triggers at certain points in the game.

Use a patched emulator, that is how retroachievements.org do it.
Early DOS games only need 640 KB RAM at best, and memory mapping is clear and fixed.
Later games may need RAM up to 16~32 MB, but it is like zero in today's system.

Windows games are much more complex, adding achievement will be much more difficult.

Not unless the rightsholder is able to put them in and chooses to do so.

Its sad, i would love to buy and play more old games with achievment. i dont know why, but its a great think to play a game more intensive and its keeping me playing a game.

Unlikely, trying to add achievements would likely require patching it into the game and that's probably not worth the effort. So, for games that they don't have source code, they probably wouldn't be able to at all. Games that do have source code available, would probably not be profitable to do so as patching in achievements would run the risk of making changes that introduce bugs or affect compatibility.

For me, achievements are modern publisher's excuse to say "you need XX hours to complete our game".
Older games were actually playable during that time.

Forgot about that, that's yet another reason why this is unlikely to happen. It might be possible to have an achievement for playtime, but that would be potentially tricky as there isn't necessarily a way of knowing about playtime versus the time the program is running for, which aren't necessarily the same thing.

I wish but nope, you will have to recode and integrate a system to the game for that to work. And a lot of the companies that made these games do not exist anymore or have the "source code" as toxicTom mentioned.

Even if they still exist, the source codes have often been lost.

That's a common use. It's also a way of encouraging players to practice various skills and techniques.

And, of course, a way of stretching the gameplay without really doing much work.

IMHO, older games don't need this as much as the gameplay was simple enough that you don't really need to have an achievement for jumping over 10 holes or collecting a thousand gems. Old school achievements were really more like Sonic the Hedgehog's super sonic or Mario's infinite lives in Super Mario World.

The client can dig into the game's internals (saves or memory). If SSI Goldboxes can get realtime maps, no way they can't get achievements either. Unless you want to see them in the game's interface, nope, it's not required. Achievements are scorekeeping, you don't need copyright for that.